The present invention relates to a circuit board on which various types of semiconductor devices and electronic components are mounted. In particular, the present invention relates to a thermal conductive substrate that is constituted by a resin substrate with high heat-radiating characteristics suitable for a power electronics field, and to a method for manufacturing the same.
Accompanying the request for high performance and miniaturization of electronic equipment in recent years, there has been a demand for high-density packaging of electronic components and semiconductors. Accordingly, it is more likely that heat generated from different components is concentrated, leading to a higher temperature. In order to release this heat from the equipment quickly, it is important to design the equipment considering heat radiation. Thus, circuit boards also need to have high heat-radiating characteristics.
In order to improve the heat-radiating characteristics of the circuit boards, there are conventional printed boards using glass-epoxy resin. On the other hand, metal-based circuit boards also have been suggested. The metal-based circuit board is produced in such a manner that circuit patterns are formed on one side or both sides of a metal plate such as copper or aluminum via an insulator layer. When still higher heat-radiating characteristics are required, a substrate that is formed by directly joining a copper plate to a ceramic substrate such as alumina or aluminum nitride is used. Generally, the metal-based circuit boards are used for applications requiring relatively small electric power. However, the insulator layer has to be thin to achieve high thermal conductivity. This causes problems in that the metal-base circuit boards are susceptible to noise between the insulator layer and the metal base and have low withstand voltage.
For the purpose of solving these problems, a substrate formed in the following manner has been suggested recently. That is, a composition produced by filling an inorganic filler having an excellent thermal conductivity in a thermosetting resin is integrated with a lead frame serving as an electrode, so as to obtain the substrate. The substrate using such composition is disclosed, for example, in JP 10 (1998)-173097 A. FIG. 6 shows the method for manufacturing this thermal conductive substrate. First, a thermal conductive resin composition slurry containing at least the inorganic filler and the thermosetting resin is formed so as to obtain a thermal conductive green sheet 61. This thermal conductive green sheet 61 is dried, then stacked onto a lead frame 62 as shown in FIG. 6A. Subsequently, as shown in FIG. 6B, the thermal conductive green sheet 61 is cured by heating and pressing so as to be an insulator layer 63 containing a thermal conductive resin cured material. In this manner, a thermal conductive substrate 64 is completed.
When producing the substrate by such a method, in order to maintain high heat-radiating characteristics and withstand voltage of the thermal conductive substrate 64 and to increase the adhesive strength of the lead frame 62 serving as a wiring pattern to the insulator layer 63, it is preferable that the thermal conductive resin composition constituting the thermal conductive green sheet 61 covers the end faces of the wiring pattern of the lead frame 62. Thus, at the time of heating and pressing, the thermal conductive resin composition has to have the fluidity necessary for covering the end faces of the pattern of the lead frame 62. On the other hand, in order to increase the thermal conductivity of the thermal conductive substrate, the higher ratio of the inorganic filler in the thermal conductive resin composition is effective. However, when the ratio of the inorganic filler increases, the fluidity of the thermal conductive resin composition decreases. This causes a problem in that it becomes difficult to cover the end faces of the lead frame with the thermal conductive resin composition. In addition, when the ratio of the inorganic filler increases, the organic constituent for maintaining adhesive characteristics between the thermal conductive resin compositions decreases. This causes a problem in that it becomes difficult to process the thermal conductive resin composition into a desired shape and to maintain and handle this shape.
It is an object of the present invention to solve the problems described above and to increase thermal conductivity of a substrate and, at the same time, maintain fluidity of a thermal conductive resin composition so as to cover the end faces of a lead frame with the thermal conductive resin composition in a portion where the lead frame is in contact with the thermal conductive resin composition, thereby maintaining high heat-radiating characteristics and reliability. It is a further object of the present invention to achieve easy handling of the thermal conductive resin composition, thereby making it easier to produce the substrate.
The above-described object can be achieved in the following manner.
First, a thermal conductive substrate of the present invention includes a first electrical insulator layer, a second electrical insulator layer, and a lead frame serving as a circuit pattern. The first electrical insulator layer is formed of a thermal conductive resin composition containing a thermosetting resin and an inorganic filler (hereinafter, also referred to as a reinforcing material), and is joined to the lead frame. The second electrical insulator layer is provided on a side of the first electrical insulator layer not in contact with the lead frame, and is formed of a thermal conductive resin composition containing the inorganic filler and a resin composition containing the thermosetting resin. The second electrical insulator layer has a higher thermal conductivity than the first electrical insulator layer.
Next, a first method for manufacturing the thermal conductive substrate of the present invention includes producing a first thermal conductive resin composition by mixing an inorganic filler and a resin composition containing a thermosetting resin that is not cured, producing a second thermal conductive resin composition having a higher thermal conductivity than the first thermal conductive resin composition by mixing the inorganic filler and the resin composition containing the thermosetting resin that is not cured, and stacking a lead frame, the first thermal conductive resin composition and the second thermal conductive resin composition in this order and integrating them by heating and pressing, as well as curing the thermosetting resin.
Then, a second method for manufacturing the thermal conductive substrate of the present invention includes processing a first thermal conductive resin composition that has been produced by mixing an inorganic filler and a resin composition containing a thermosetting resin that is not cured into a sheet, producing a second thermal conductive resin composition having a higher thermal conductivity than the first thermal conductive resin composition by mixing the inorganic filler and the resin composition containing the thermosetting resin that is not cured, processing the second thermal conductive resin composition into a layer having a substantially constant thickness and adhering the layer onto a heat-radiating board, and stacking a lead frame, the first thermal conductive resin composition and the heat-radiating board provided with the second thermal conductive resin composition in this order so that the first and second thermal conductive resin compositions contact each other and integrating them by heating and pressing, as well as curing the thermosetting resin.
Next, a third method for manufacturing the thermal conductive substrate of the present invention includes processing a second thermal conductive resin composition produced by mixing an inorganic filler and a resin composition containing a thermosetting resin that is not cured into a layer having a substantially constant thickness and adhering the layer onto a heat-radiating board, processing a first thermal conductive resin composition produced by mixing the inorganic filler and the resin composition containing the thermosetting resin that is not cured and having a lower thermal conductivity than the second thermal conductive resin composition into a layer having a substantially constant thickness, and adhering the layer onto the second thermal conductive resin composition that has been adhered onto the heat-radiating board in the first step, and stacking a lead frame and the heat-radiating board provided with the first and second thermal conductive resin compositions so that the lead frame and the first thermal conductive resin composition contact each other and integrating them by heating and pressing, as well as curing the thermosetting resin.
In the present invention, the lead frame is formed by arranging a wiring pattern on a metal plate. Leads (terminals) are connected to each other by a frame so as to be formed into one component. After the formation, the frame portion is cut off, so that the terminals are insulated electrically. The heat-radiating board is a plate provided to the substrate for the purpose of promoting a radiation of heat generated in elements on the substrate out of an appliance and dissipating the heat quickly to a large area, thereby preventing a local temperature increase. In many cases, the heat-radiating board is made of metal and also serves as a structural reinforcement and a ground connection. In addition, the inorganic filler in the present invention is a powder made of an inorganic substance and has an effect of providing a function (heat-radiating characteristics) when filled into a resin matrix.